This invention relates to an improvement in phase zone plate optics embracing contact and intraocular lenses. A "phase zone plate", as employed herein and in the claims, is a unitary optical region of a lens utilizing the combination of a zone plate and optical facets (such as in the form of echelettes) in the zones of the zone plate, and the combined facets in the zones diffract light to produce a specific wavefront which results in a specific intensity distribution of light at a variety of orders (e.g., 0.sup.th,1.sup.st, etc.) of the zone plate. The orders constitute the foci of the zone plate. In a restrictive sense and also in the most utilitarian sense, the phase zone plate is designed for general lens applications where the distribution of light at effective intensities is dependent upon zone spacing for yellow light. Yellow light, as employed herein, is that portion of the visible spectrum at 530-570 manometers.
The invention relates inter alias to contact lenses. Contact lenses are classical vergence type lenses. They possess a concave corneal bowl (the posterior surface) that allows fitting to the eye and the outer surface (the anterior surface) is smooth and shaped to allow the eyelid to slide over the eye and to provide proper vergence of light (taking the lens material's refractive index into consideration) to a focal point accommodating to the eye. The majority of the commercial contact lenses are shaped such that the lenses are thinnest about the optical axis and the depth of the lenses gradually increases along a sloped radial length extending in the direction of the lens perimeter. Owing to the difference in depth extending from the optical axis, light passing through the optical axis has to pass through less of the lens material. Because light travels faster in air, the light passing through greater depth relative to light passing through lesser depths will be shifted, hence be retarded in time..sup.1 Consequentially, the shape of the lens is selected to accommodate this progressive retardation of the light so that the lightwaves emanating from the posterior surface are in synchronization in reaching a desired focal point. FNT 1. See Fincham et al., Optics, published by Butterworths, London, 9.sup.th edition, 1980, 1981, pp. 72-75.
This invention concerns contact lenses utilizing phase zone plate optics, such as phase zone plate bifocals and "tuned" Fresnel lenses making use of concentric annular zones. Such lenses generally follow the designs described, for example, by Allen L. Cohen in U.S. Pat. Nos. 4,210,391; 4,338,005; and 4,340,283 ("Cohen patents"). A Cohen lens design provides that the radii "r.sub.n " of the annular and concentric zones are substantially proportional to .sqroot.n and that the zones are cut so as to direct light to more than one focal point.
Prior to the work of Cohen, the analyses provided by Fresnel and Wood demonstrated that every full-period zone of a phase zone plate comprises two half-period zones (odd and even zones) which are sufficiently out of phase to destructively interfere at the 1.sup.st order focal point. They taught that by blocking an odd or even zone out or phase shifting the light from an odd to even zone, destructive interference is minimized. However, this did not result in a useful bifocal lens. Cohen found that by making the odd and even zones different and thereby directing light differently, the incident parallel light passing through the zones would be coordinated to form a diffraction limited element that directs the light to different focal points thereby providing a useful multifocal lens. The differences in the odd and even zones are accomplished by variations in thickness or refractive index in and between the zones.
The Cohen lens design with phase zone plate optics allows bifocal lens constructions which are exceptionally thin. Contact lenses may be designed with phase zone plate optics in order to achieve a bifocal or other multifocal effects. The specific chromatic properties of a phase zone plate may be incorporated in the design of a contact lens including a contact lens having multifocal properties. All phase zone plate optical elements which are designated bifocals are possessed inherently with the ability to focus light to more than two focal points. They are designated bifocals because the intensity levels of the light to any two orders, e.g., the 0.sup.th and 1.sup.st order focal points are adequate for bifocal applications. In that sense, every bifocal distributes light to a third, and possibly more, focus. The judgment of whether a lens is a bifocal or trifocal is not base on any strict rule. If the wearer of the lens does not find objectionable the presence of the third or more focuses, then the lens is probably adequate as a bifocal..sup.2 FNT 2. See Klein and Ho, SPIE, August 1986, Table 2 and the comments about Table 2.
Other references mentioning or suggesting phase zone plate optics in regards to contact lenses are G. Forst, "Research into the Usability of Circular Grids as Aid to Vision," Der Augenoptiker, 1966 (12), pages 9-19; Ziegler, "Fabrication or Correction of Optical Lenses," as modified by Cohen, see column 4, lines 27-36 of Cohen, U.S. Pat. No. 4,339,005, and column 5, line 63 to column 6, line 68, Cohen, U.S. Pat. No. 4,210,391; Freeman, U.S. Pat. No. 4,637,697; and Freeman, U.S. Pat. No. 4,642,112 (to the extent that holography embraces phase zone plate optics).
Bifocal contact lenses utilizing the above principles of phase zone plate optics are commercially available. Such lenses are believed to utilize parabolically-profiled, stepped annular facets each comprising a full-period zone where each zone has a depth of an optical path length of .lambda./2, providing a physical depth of .lambda./2 (.eta.'-.eta.). .eta.' and .eta. are the indices of refraction of the lens and the medium (e.g., lachrymal layer) in which the lens is interacting and .lambda. is the design wavelength, in this case that of yellow light. This results in a bifocal contact lens where the 0.sup.th and 1.sup.th orders have an equal split of yellow light intensity at about 40.1%.
A full-period zone, for purposes of this invention, is defined as the smallest repetitive sequence of facets within a phase zone plate which are spaced substantially proportional to .sqroot.n. Such spacing is characterized by the formula: ##EQU1## where d represents the 1.sup.st order focal length. A half-period zone, for the purposes of this invention, is characterized by the formula: ##EQU2## where d represents the 1.sup.st order focal length.
If one were to alter the depth of the steps of this lens design there would result in vastly different concentrations of intensity of the incoming light to the focal points. For example, a phase zone plate with full-period spacing of the echelettes where the depth of the step of each echelette is .lambda., and the profile of the echelette is parabolic, using the wavelength of yellow light as the design wavelength, the resulting lens is essentially monofocal to the 1.sup.st order to the user regardless of the shape of the carrier lens overall. As the depth of this step is reduced, the flattening out of the echelettes results in different variations in depth over this spacing consequently effecting the differential shaped odd and even zones of Cohen. With a depth greater than .lambda./2, the intensity of light at the 1.sup.st order is significantly greater than at the 0.sup.th order because the thickness differences between the odd and even zones are high. It is only when the depth of the echelettes are .lambda./2 that the lens provides uniform light intensities at the 0.sup.th and 1.sup.st orders. Here the thickness differences between the odd and even zones are balanced. When the depth of the echelettes fall below .lambda./2, the thickness differences between the odd and even zones becomes minimized and this results in greater light intensity at the 0.sup.th order.
What is lacking in the art is flexibility to vary the depth of the steps yet retain a desirable split of the intensity of the light to the 0.sup.th and 1.sup.st orders or any other multiple order combinations. It would be desirable to have the ability of generate multifocal, especially bifocal, lenses in which the depth of the echelette may be a variable factor in lens design, yet the lens designer has the capacity to make a lens which has a favorable split of light to the 0.sup.th and 1.sup.st orders or any other combination of orders.
There is described herein a lens design which employs phase plate optics yet is freed of the constraints of facet structure to the parabolic profile. The lens design of this invention allows the lens manufacturer to make a lens, especially a contact lens, that utilizes a wide selection of echelette configurations yet allows the manufacturer to achieve a desired split of light to the near and far focal points. This flexibility in lens design frees the lens manufacturer from such constraints in lens design as echelette depth, traditional parabolic or flat profiles, and the like considerations.